"Two behavioral goals are achieved simultaneously during
forward trunk bending in humans: the bending movement per se and
equilibrium maintenance. The objective of the present study was
to understand how the two goals are achieved by using a
biomechanical model of this task. Since keeping the center of
pressure inside the support area is a crucial condition for
equilibrium maintenance during the movement, we decided to model
an extreme case, called "optimal bending", in which the
movement is performed without any center of pressure displacement
at all, as if standing on an extremely narrow support. The "optimal
bending" is used as a reference in the analysis of
experimental data in a companion paper. The study is based on a
three-joint (ankle, knee, and hip) model of the human body and is
performed in terms of "eigenmovements", i.e., the
movements along eigenvectors of the motion equation. They are
termed "ankle", "hip", and "knee"
eigenmovements according to the dominant joint that provides the
largest contribution to the corresponding eigenmovement. The
advantage of the eigenmovement approach is the presentation of
the coupled system of dynamic equations in the form of three
independent motion equations. Each of these equations is
equivalent to the motion equation for an inverted pendulum.
Optimal bending is constructed as a superposition of two (hip and
ankle) eigenmovements. The hip eigenmovement contributes the most
to the movement kinematics, whereas the contributions of both
eigenmovements into the movement dynamics are comparable. The
ankle eigenmovement moves the center of gravity forward and
compensates for the backward center of gravity shift that is
provoked by trunk bending as a result of dynamic interactions
between body segments. An important characteristic of the optimal
bending is the timing of the onset of each eigenmovement: the
ankle eigenmovement onset precedes that of the hip eigenmovement.
Without an earlier onset of the ankle eigenmovement, forward
bending on the extremely narrow support results in falling
backward. This modeling approach suggests that during trunk
bending, two motion units - the hip and ankle eigenmovements -
are responsible for the movement and for equilibrium maintenance,
respectively."